||Sulfate in atmospheric particles has been found to be correlated with excess mortality rate and it is well known that sulfate particles are acidic. This thesis investigates the aerosol acidity, which is likely a better causal agent of health effects, in PM2.5 in Hong Kong. The strong acidity and the concentrations of ammonium, sodium, sulfate, nitrate and chloride in PM2.5 in Hong Kong, using a Harvard honeycomb denuder/filter-pack system were measured at the different sites (HKUST, TST and Ho Man Tin) in Hong Kong. Overall, the strong acidity of PM2.5 in HK is higher than that reported elsewhere (maximum of about 180 nmol/m3 in this study). Using back trajectory and mixing height analysis, we estimated that 40% of the measured sulfate and ammonium of PM2.5 in Hong Kong is from non-local sources. The sampling of acidity and semi-volatile species such as ammonium, nitrate and chloride is complicated by artifacts. Various reactions between gas and particles and among the collected particles lead to sampling errors. We examined the role of aerosol composition in the sampling artifacts of aerosols. The results show that the analysis of the artifact formation can be categorized into two regimes; as ammonium rich (AR) with the molar ratio [NH4+]/[SO42-] ≥ 1.5 and as ammonium poor (AP) samples with the molar ratio [NH4+]/[SO42-] < 1.5. The AR samples were characterized by high nitrate concentrations and low in-situ free acid; the evaporation of HNO3 and HC1 with concomitant evaporation of NH3, was more important in the sampling artifact of nitrate and chloride. The AP samples were characterized by low nitrate concentrations and high in-situ free acid; the evaporation of HNO3 or HCl alone was the important parameter in controlling the extent of sampling loss. We have also developed a methodology to estimate the contribution of each artifact reaction in the sampling artifacts of nitrate, chloride, ammonium and acidity. We extended our analysis to a much larger database, that is the 24-hr Respirable Suspended Particles (RSP: PM10) data set from seven air-quality-monitoring sites run by the Hong Kong Environmental Protection Department in 2001, to estimate the aerosol acidity of PM2.5, which is not regularly monitored. In-situ free acid concentrations and pH of aerosols, which are more relevant to environmental impact of aerosols than their strong acidity, were estimated. Because of the small variation in ambient temperature in HK, the acidity characteristics of fine particles are a function of mainly the relative humidity and the ammonium to sulfate ratio. Overall, the fine mode particles are highly acidic with average molar [NH4+]/[SO42-] ratio of 1.25, strong acidity of 66.3 nmol/m3, in-situ acidity ([H+]free) of 21 nmol/m3, and in-situ pH of -0.05. Our findings suggest that even the more neutralized ([NH4+]/[SO42-] >1.5) particles, such as those found when HK is under the influence of continental air masses from the Chinese mainland, can have high in-situ acidity and low pH when the RH is low. This study calls for more investigations of the acidity of aerosols in HK.